1. Field of the Invention
The present invention relates to light measuring devices for use in exposure meters and to such light measuring devices which are assembled into auto exposure cameras for photography.
2. Description of the Prior Art
Light measuring devices generally used for photography generate exposure data signals (e.g., the combination of an aperture size and an exposure time) in response to a light measuring output representative of some average of the various luminance values distributed within a light measuring range, whereby a part of an object having a luminance value corresponding to the average value is reproduced in an approximately middle density of the reproducible density on a film (i.e., inside the film latitude). The combination of aperture size and exposure time is automatically determined by a camera in response to such an exposure data signal, or a photographer reads the display value of the exposure data signal and manually sets the exposure data for photography. Consequently, part of the object having a luminance value greater than the average value is reproduced at a higher density on the film (i.e., brighter when seen through the positive), while another part of the object having a luminance value smaller than the average value is reproduced at a lower density on the film (i.e., darker when seen through the positive). The above-described conventional exposure value determination method is based on the principle that the average of the luminance values is correlated with the center of a film latitude so that a majority of luminance values for various parts of the object are expected to be reproduced on a film in the corresponding density distribution, since the majority of luminance values disperse around the average value. The so-called correct exposure in the prior art is thus determined.
In fact, however, the above-mentioned conventional exposure value determination method does not always provide correct exposure. When the luminance distribution of an object is, for example, wider in range than the film latitude, parts of the object with less luminance than a lower limit appear black in the shadow of almost uniform density in the positive print, according to the conventional exposure determination method, although they originally have a variety of density distributions. However, parts of the object with more luminance than an upper limit turn white and disappear in the highlight of almost uniform density in the positive print, and their luminance distribution is not reproduced. If, under this condition, a photographer intends to reproduce a relatively low luminance range of the object or a relatively high luminance range of the object on a film, such results no longer correspond to correct exposure, despite the "correct" reproduction of a middle luminance range of the object.
Furthermore, when the luminance distribution of an object is narrower than the film latitude, the object is generally reproducible as an overall bright picture or an overall dark picture, depending on the composition intended by the photographer. According to the conventional exposure determination method, however, an object is always reproduced as a picture of neutral brightness regardless of the intention of the photographer, resulting in an exposure which is not correct. (A negative film permits the addition of a composition intended by the photographer during printing, whereas a reversal film does not.)
In such a case, correct exposure is not obtainable from a "so-called correct exposure value". Therefore, photography may be performed with an exposure value which is intentionally shifted from the "so-called correct exposure value". However, the amount it is shifted depends entirely upon the experience of an individual photographer, ensuring no reproduction of a luminance distribution as intended.
Moreover, there is a photographic spot measurement known as a method for obtaining detailed luminance information of an object, in which the luminance of only a limited area of the object is measured. According to the spot measuring method, it is possible to measure not only an average luminance of the entire object but also the luminance of the individual parts of an object, thereby providing luminance information of any desired part alone, as well as the differences in luminance between the darkest and brightest parts.
With photographic spot measurement, however, an exposure data signal is generated so that the luminance of the measured part of an object may be reproduced almost in the middle density within the film latitude. Thus, although different in that the light measuring range is narrow, the above-described photographic spot measurement method is substantially based on the technical concept identical to that of the so-called correct exposure value determination method using an average luminance of the whole object. Specifically, the part to be spot-light-measured is reproduced in the middle density, a part with more luminance than the spot-light-measured part being reproduced at higher density and another part with less luminance at a lower density. Thus, these two methods are apparently based on the same technical concept when the "luminance of a part to be spot-light-measured" reads as the "average of luminance of the whole object".